a) Field of the Invention
The present invention relates to a method of making resist patterns, and more particularly to a method of making resist patterns by changing resist patterns after development.
Degree of integration and pattern sizes of semiconductor integrated circuits (IC) are becoming more serious and critical. IC manufacturing technique demands for patterns of finer size. 64 Mb DRAMs require a pattern size of 0.3 to 0.4 .mu.m.
b) Description of the Related Art
A resist pattern is generally formed by selectively applying ultraviolet rays to a coated resist film to expose it and by selectively developing and removing the exposed or unexposed regions of the resist film. Exposure is performed by reducing the size of a pattern on a mask (reticle) by, for example, 1/5 by a projector lens and focusing the reduced pattern on a resist film. A resolution is determined by a wavelength of ultraviolet light, an numerical aperture of a projecting lens, and other parameters.
In order to improve a resolution, either a large numerical aperture or a short wavelength is used. However, a large numerical aperture makes a depth of focus shallow. A depth of focus is required to be deeper than a certain value in order to realize high precision exposure on a substrate having steps.
The remaining approach is to shorten the wavelength of ultraviolet light. The wavelength of exposure light has shortened from g-line to i-line of mercury, and from KrF laser beams to ArF laser beams. The more the wavelength becomes short, the more the conditions to be satisfied by optical elements and other devices become severe.
Another approach to improving a resolution is phase shift exposure. Light having a different phase from an exposure main beam is superposed on a pattern edge portion extending by diffraction of the main beam. Interference of the light with the main beam reduces a light intensity so that an unsharp pattern can be avoided. The structure of a mask, however, becomes complicated because it is necessary to use two or more beams having different phases.
As described above, although finer resist patterns have been desired, it is not so easy to realize such patterns.